Astrophotography with a simple compact camera

Most of us now live in urban areas where the wonders of the night sky can hardly be seen, owing to light pollution. Those who live deep in the countryside, well away from street lights, can see much more. I have taken comparison photos that give some idea of the difference: click to view.

Most people do not own a Single-Lens Reflex (SLR) camera, with interchangeable lenses. Instead they have what is generally known as a compact camera.

So let's see what astrophotography can be done with those limitations: using a compact camera from a light-polluted site.

Equipment

I decided to demonstrate by using a Canon PowerShot S40 camera. I bought this in 2003 to have something small that I could always carry around because my SLR kit, with several lenses, is bulky and heavy. A 7-year-old camera is way behind the times in its capabilities. Its photos have only 4 megapixels. However it does have certain capabilities that are really a minimum requirement for photographing the night sky:

Autofocussing can be switched off. There is a button marked MF (Manual Focus) and then the zoom control sets the focus - I slid it to the infinity end of the scale shown on the back of the camera, on its LCD screen.

There is an M (for Manual) position on the main setting dial on top of the camera. This enables exposure time, aperture, and ISO sensitivity to be adjusted separately, again via the display screen.

The menu allows setting RAW image format instead of JPEG. If you want to photograph stars fainter than can be seen with the naked eye that is essential.

If you are buying a camera with a view to trying some astrophotography those are important requirements.

Next the camera needs to be fixed to something. In principle you could just lie it on its back pointing upwards. But you must position it so that pressing the shutter will not move it at all. Compact cameras do not have cable (or wireless) shutter release devices, so you have to touch the camera in order to open the shutter. The best way to hold the camera fixed is to use a tripod of course. Just about all cameras do have a screw thread underneath for attaching to a tripod. A cheap tripod will do, as long as the camera does not wobble when you press the shutter.

Another possibility is to set a time delay for each exposure so that any wobbling has settled down by the time the shutter opens. Yet another approach is to use a piece of black (non-reflective) card in front of the lens, removed only after pressing the shutter, but then it is difficult to ensure equal exposure times for the multiple exposures we will be using.

Here is the arrangement I used for for the following demonstration: my "ancient" Canon S40 on a Manfrotto tripod.

Taking the photos

I set the camera for

manual focus, as described above;

the highest possible sensitivity (a paltry ISO 400 - nowadays you will almost certainly be able to go higher);

the longest possible exposure time, 15 seconds (marked as 15" on the LCD screen); and

the maximum aperture, f/4.0 (you might try reducing that if focus is not sharp).

(If you are not sure what those factors mean, and how they trade off against each other, read my camera techniques page.)

Note that, because you have to touch the shutter button, it is best to set a timed interval even if the camera has a B (for Bulb) setting.

So then I took 20 shots in quick succession. That really meant about 30 seconds apart because the camera spent about as long as the exposure time processing after each picture was taken.

Here is the first of those 20 photos:

Canon Powershot S40 15s f/4.0 ISO400 2010-08-30

That is certainly not very impressive, but we are not finished yet. Combining all 20 shots will make a much better result.

The area shown covers the constellations of Cygnus (the cross shape), Lyra beside it on the right (the bright star is Vega), below Cygnus the small arrow-shaped Sagitta, and some of Aquila (the bright star at the bottom is Altair). Here is a cropped portion from the photo, just covering Vega and its constellation, Lyra:

Canon Powershot S40 15s f/4.0 ISO400 2010-08-30

This is at 1:1 scale by which I mean 1 pixel on the screen corresponds to 1 pixel in the camera (whereas the full photo above was scaled down to fit in 500 x 667 pixels, from an original size of 1704 x 2272 - remember this is an old camera having only 4 megapixels).

Notice in particular in this enlarged portion that star trailing is not evident. Our exposure time of 15s was short enough to avoid trails. It would probably not want to be any longer though.

Processing

Using the image processing software GRIP (free from this site) we can combine all 20 photos. The first thing we might try is simply adding them together, as if they were stacked one on top of the other. This can be done from the batch menu of GRIP, shown on the right. Either the averaging or the accumulating option would work but the latter enables more to be got out of the stacked image when it comes to enhancing it.

Below is the result of accumulating my 20 photos. Don't worry about the trails; we will see how we can get rid of them!

Even though each individual photo showed no noticeable star trails, the whole sequence took more than 10 minutes and so the Earth's rotation has produced a significant effect, spoiling the image really.

So instead, we use a different option on GRIP's batch menu (see above). Near the top of the menu is "Astro combine into 1 image". That first displays a fairly complicated dialogue.

For now, we simply use the "Browse" button at top right to select the files to process and then click the "Process" button at the bottom. GRIP then finds the brightest stars in each image and matches the geometrical patterns that they make, across the images. It is then able to transform (warp) each image to superimpose the stars exactly. This time the result is clearer:

Canon Powershot S40 20x15s f/4.0 ISO400 2010-08-30

Now we can see much more detail than was evident in a single frame. Even the Milky Way is starting to be visible, down through the centre of Cygnus. The darker edges of the image are because those parts were not covered by all 20 photos. I would normally crop the dark part off (an option in the Geometry menu of GRIP).

Here is a cropped portion covering Lyra again at 1:1 scale, for comparison with the cropped portion of a single frame shown earlier.

Canon Powershot S40 20x15s f/4.0 ISO400 2010-08-30

This shows stars down to magnitude* 8, which is less than one sixth of the brightness of the faintest stars that can be seen by the naked eye even from an unpolluted location. A longer sequence than 20 photos would show even more.

I want to emphasise that this was taken with an old 4-megapixel compact camera, held fixed on a simple photographic tripod, at a light-polluted location (Tyneside, UK). There was no motorised drive or fancy guiding system to follow the rotation of the Earth, nor any telescope to collect extra light or magnify the view.

We will go on to see how telescopes and motorised tripods can help but GRIP was designed to make it possible to do astrophotography without all that expensive gear.

Example photos

Here are some more examples of astrophotos taken with a fixed camera (albeit SLR) and processed with GRIP:

A note about magnitudes

* Magnitude is a logarithmic scale. An increase of 5 magnitudes is a 100-fold decrease in brightness, so 1 magnitude is a factor of just over 2.5. Under good conditions it is generally reckoned that magnitude 6 stars are the faintest visible to the naked eye.

How do I know that the cropped photo above shows stars down to magnitude 8? Because GRIP can also draw star charts, using data from Hipparcos and Tycho space projects (data files downloadable from here). The charts are interactive: as the mouse moves over them they show the id and magnitude of the nearest star, as in this example which covers Lyra again: